The NYPD yesterday took aim at the cop who fired the first of 50 shots at unarmed groom Sean Bell in 2006, officially blaming the officer for triggering the mayhem that killed Bell and stained the department.

Detective Gescard Isnora’s conduct was “outrageous, unimaginable and unjustified” when he opened fire on a car carrying Bell and two friends who had just left Bell’s bachelor party at a Queens strip club that November morning, said NYPD lawyer Adam Sheldon at a departmental hearing that began yesterday.

Isnora, who fired 11 times, has been acquitted of criminal charges in the case, as have Detectives Michael Oliver and Marc Cooper.

But the departmental hearing for Isnora and a fourth cop involved, Michael Carey, will now determine whether the pair violated any NYPD regulations, and, if so, how they will be punished.

Oliver, who fired 31 bullets, still faces departmental charges. Sources said the 17-year veteran is being pressured to accept a deal forcing him to retire now, meaning he would get a reduced pension in three years, after his 20-year anniversary with the department.

He also would be denied what’s known as a “good-guy letter’’ from the department allowing him to continue to carry a gun as a civilian.

Cooper, a 23-year veteran, has agreed to retire with his full pension, according to his attorney.

Shelton told the judge that Isnora’s “error in judgment facilitated everything that happened after that.

“The problem is he never saw an actual threat coming from the car. He couldn’t have because one didn’t exist,” the lawyer said.

Carey fired three shots, but was never criminally charged and faces only the departmental scrutiny.

The NYPD said Isnora and Carey face internal charges involving firing their weapons outside department guidelines. Isnora, who was working undercover, was also hit with a violation for blowing his cover.

The cops said they believed that someone in the car had a gun and that Bell was trying to run them over.

Isnora’s lawyer, Philip Karasyk, said that if Bell “had just put his foot on the brake instead of the accelerator, none of us would be here today.”

The Justice Department dismissed a civil-rights claim against the officers last year.

But the city still wound up shelling out more than $7 million to settle a federal lawsuit brought by Bell’s fiancée, Nicole Paultre-Bell, who had two young children with Bell and took his name after his death.

“My children are one of the main reasons why I have been standing up for it since Day One,” Paultre-Bell said outside yesterday’s hearing.